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Quarterly Monitoring and Sampling Activities March 18, 1993 Southland Site No. 20304, 455 Grant Line Rd., Tracy, CA 2 Monitoring Well Sampling After well monitoring, groundwater samples were collected from the 14 monitoring wells according to Groundwater Technology, Inc., Standard Operating Procedures (SOPs) (Attachment 3). The ground- water samples, a trip blank, and a rinsate blank were submitted under chain-of-custody protocol to GTEL Environmental Laboratories (GTEL) in Concord, California. Each sample was analyzed for benzene, toluene, ethylbenzene, and xylenes (BTEX), and for total petroleum hydrocarbons-as-gasoline (rPH-G) using Environmental Protection Agency (EPA) Methods 5030/8020 and 8015. Twelve samples were also analyzed for fecal coliform bacteria. Figure 2 illustrates the distribution of benzene and TPH-G. Table 2 summarizes the analytical results of this sampling event and the historical analytical data. Fecal coliform bacteria were detected in groundwater samples from four wells (MW-1, MW-2, MW-8, and MWA5), with the highest concentration (less that 23.0 organisms per 100 milliliters) near well MW-8. The laboratory analytical reports and the chain-of-custody records for the December 28, 1993, samples are included in Attachment 4. Conclusions and Recommendations ■ Historically, wells MW-2 and MW-3 reported hydrocarbon concentrations below method detection limits. Detection of hydrocarbons in wells MW-2 and MW-3 during the December 28, 1992, sampling may be because more sensitive detection limits were provided by GTEL and the rising groundwater levels. 0 The anomalous groundwater elevations in wells MW-9 and MW-10 may be caused by the shallower screening interval used in these two wells. The different elevations measured in wells MW-11 and MW-2 may be because of changes in subsurface lithologies and/or man-made conduits affecting groundwater elevations. ■ The presence of coliform bacteria appears to be associated with a former sewer leak near wells MW-8 and MW-9. Overall the concentrations appear to be dropping. Off-site concentrations may be because of leaks from the public sewer system. Based on the groundwater monitoring and analytical data collected to date, Groundwater Technology recommends that quarterly monitoring continue during 1993. Proposed groundwater remediation will reduce migration of the dissolved-phase hydrocarbon plume. r R3767A1.MK p J, GROUNDWATER '�J(jI TECHNOLOGY,INC.